The present invention relates to blow molding apparatus used to mold hollow bodies from plastic material by blow molding in which two molds are clamped in a back-to-back relationship. The invention is directed especially to an extrusion blow molding apparatus in which both halves of each mold are moved during closing and opening of the molds.
In blow molding, a parison, i.e., a continuous tube of plastic material, is first extruded between two mold halves or sections, each having a back surface and a front surface, with a segment of the desired mold cavity cut into the surface of each section front surface. The opposed surfaces of the mold sections are positioned in parallel planes.
After extrusion of the parison, the mold segments are clamped together so tat the front surfaces of the mold sections are held against each other to form the complete mold cavity, and air is injected into the interior of the parison, e.g., with a blowing needle, pin or other apparatus, which penetrates the parison wall. Injection of air into the interior of the parison expands the parison to conform to the shape of the mold cavity. After cooling, the mold is opened and the hollow body ejected.
A typical blow molding apparatus is comprised of one or more clamps having opposed platens supporting mold sections. A toggle linkage, or other type of platen positioner means, is operated by some type of actuator to move the platens between open and closed positions along tie bars. The linkage also includes a locking means to lock the linkage and dies in the closed position. These basic elements, however, can be configured into different types of blow molding apparatus, depending upon the end result desired.
For example, a blow molding apparatus known as a continuous rotary blow molding apparatus is formed by positioning a plurality of outwardly facing mold clamps in a circle on a rotary support wheel, comprised of a pair of spaced, parallel, vertically positioned disks. Upon rotation, each clamp, while in an open position, passes an extruder which extrudes a parison between spaced mold sections. Thereafter, the clamp, actuated by the platen positioner, closes the mold sections around the parison. An air injector, such as a blowing needle then injects air into the interior of the parison inside the mold cavity to expand the parison into a hollow body conforming to the interior shape of the mold cavity. The clamp then continues its rotation to cool the hollow body. After the hollow body is cooled, the mold clamp is opened and the hollow body ejected. Each clamp supported on the rotary wheel continues in sequence through these molding stages.
In another kind of blow molding apparatus, known as a shuttle molding apparatus, mold sections are supported in an open position in a single clamp which is moved back and forth between a molding station and a discharge station. The clamp is first moved to an extrusion or molding station where a parison of polymeric material is extruded between the mold segments. The mold segments are then clamped about the parison, air is injected into the parison to form the hollow body, and the clamp is then moved to a discharge station where the clamp is opened and the hollow body discharged. The clamp is then returned with the mold sections in an open position back to the extrusion station.
In the foregoing types of blow molding apparatus, as well as other types of extrusion blow molding apparatus known in the prior art, heavy clamping structures and high clamping pressures are required in order to squeeze and pinch the parisons during clamping, and withstand the internal pressures generated within the mold cavity during injection of air to form the hollow bodies. These pressures may be, for example, on the order of from about 40 to about 125 pounds per square inch. As a result of the high pressures required, the total surface area of the mold cavity, and thus the number or size of the items which can be formed during one molding cycle, is limited.
To increase capacity, it has been necessary to either utilize larger, multiple cavity, side-by-side molds, which have required heavier platens, heavier platen actuator means and greater forces, with consequently greater energy requirements, to move the heavier clamping mechanisms, or to utilize more molding apparatuses, which has required a duplication of all parts and increased energy due to the necessity of clamping and moving more molds.
Various types of blow molding apparatuses, and blow molding sections of injection blow molding apparatus, are described in the prior art. One of the earlier devices, described in U.S. Pat. No. 3,364,520 to Hestehave, is a rotary blow molding apparatus with a plurality of molds, each clamped between movable platens, which are opened and closed by toggle mechanisms in turn operable by hydraulic cylinders.
U.S. Pat. No. 3,936,521 to Pollock et al describes a rotary blow molding apparatus having a plurality of mold clamps spaced around a rotary table. In each clamp, a movable platen holding one mold segment is moved between open and closed positions relative to a second platen holding an opposed mold segment by a linkage assembly operated by a hydraulic cylinder.
In other devices, such as the rotary blow molding apparatus described in U.S. Pat. No. 3,985,485 to Farrell, and the blow molding section of the injection blow molding apparatus described in U.S. Pat. No. 4,818,213 to Roy, the movable platens are operated directly by hydraulic cylinders.
U.S. Pat. No. 4,457,689 to Aoki, U.S. Pat. No. 4,747,769 to Nakamura et al, and U.S. Pat. No. 4,859,170 to Aoki describe rotary injection stretching blow molding apparatus in which each mold station includes two radially spaced molds. The opposed segments of each mold are supported on movable platens which are moved together with hydraulic cylinders to close the mold.
In the above prior art apparatus, an increase in capacity either requires the use of greater pressures, resulting in the need for heavier equipment and greater use of energy, or a greater number of clamping stations. U.S. patent application Ser. No. 08/252,685 to John F. Alred, Jr., filed Jun. 6, 1994, now U.S. Pat. No. 5,551,862 Sep. 3, 1996, and assigned to Wilmington Machinery Incorporated, the assignee of the present invention, the application being incorporated herein by reference in its entirety, describes a blow molding apparatus that permits molding of a greater surface area without increased pressures and energy expenditure, or the use of a greater number of clamping stations.
The Allred apparatus achieves these objectives with a clamp that stacks two molds in back-to-back relationship. The clamp includes two spaced, movable outer platens supporting inwardly facing mold halves, and a central, substantially fixed central platen supporting outwardly facing mold halves on each side. An extruder of conventional design extrudes a first parison between mold halves supported by the central platen and the first movable platen, and a second parison between mold halves supported by the central platen and the second movable platen.
After the parisons are extruded between the spaced mold halves, a platen positioner moves the first and second movable platens toward the central platen to close the molds. The platen positioner may comprise a linkage including a toggle mechanism to hold the dies in a closed, locked position during the molding stage. Alternatively, the platen positioner may comprise extendable rods, or other adjustment means, controlled by electrical, hydraulic, pneumatic, or mechanical devices, of the kinds previously disclosed in the art.
In order to ensure that both molds are subjected to equal pressure, the central platen in the Alfred apparatus is resiliently positioned by a platen retainer that allows the central platen to adjust to a limited extent toward either the first or second movable platen in response to unequal pressures or movement against the opposed mold supporting surfaces of the central platen. After the molds are closed and locked, air is blown into the interior of the parisons in the molds to expand the molds to the shape of the cavity. Thereafter, the platen positioner reverses direction to open the molds and discharge the blow molded products.
The Alfred apparatus overcomes some important limitations of prior art devices and is suitable for many molding operations. However, the structure of the apparatus limits the speed at which the molds can be opened and closed, resulting is a corresponding limitation on the number of items tat can be molded during a given time period. The cycle time, i.e., the time required to complete one molding cycle, determines the number of articles that can be produced with one blow molder in a given time. Reduced cycle times result in greater efficiency and lower cost. Thus, a blow molding apparatus having the advantages of the Alfred apparatus in which the cycle time could be reduced by opening and closing the molds with greater speed, would be of considerable commercial value.
Thus, it is a primary aspect of the present invention to provide an apparatus of the type described by Alfred that will permit faster mold opening and closing. Specifically, it is an aspect of the invention to provide a dual parison blow molding clamp supporting two molds in back-to-back relationship, including means for rapidly opening and closing the facing halves of each mold. It is another aspect of the invention to provide a dual parison blow molding apparatus including a plurality of molds of this construction. Other aspects of the invention will be apparent to one skilled in the art from a reading of the description of the invention which follows.